1. Field of the Invention
The present invention relates to an image forming device, an image forming method, and a recording medium for stabilizing an output characteristic of an output apparatus.
2. Description of the Related Art
Recently, print on demand (hereinafter, referred to as “POD”) using high-speed combination devices has become widespread in printing industries and copying industries. Efforts to put POD into widespread use have been made in order to print meeting materials or proposals in-house in general companies, and in order to perform pre-press of produced materials or printing of color comprehensive layouts at design offices. These industries have led the workflow of digital printing to be penetrated, and the convenience and cost efficiency thereof have been widely recognized. Accordingly, a “digital commercial printing” market has been formed. Printing devices suitable for the market are printing devices not only that output printing materials having a high quality level, but also that ensure a high productivity.
An electrophotographic method is known as an image recording method used for image forming devices such as copiers. In the electrophotographic method, a latent image is formed on a photoconductor drum by using a laser beam, and the latent image is developed using a charged color material (hereinafter, referred to as “toner”). The image developed using toner is transferred and fixed onto to transfer paper, thereby recording the image.
The electrophotographic method has been employed not only for printing devices serving as combination devices at offices, but also for printing devices that generate printing materials having a high quality level in the “digital commercial printing” market these days.
In the electrophotographic method, a calibration process is a process that is performed in order to allow output images to have a predetermined density quality level. The calibration process is a process that is performed in order to reduce a change in density or color taste of output images that is caused by a change in output density level due to a change over time in printing devices. A general calibration process in the electrophotographic method is described with reference to the drawings. FIG. 10A is a flowchart of an operation flow of the calibration process. FIG. 10B shows times at which the calibration process is performed. In FIG. 10B, the horizontal axis represents time. As shown in FIG. 10B, generally, the calibration process in the electrophotographic method is performed at times at which a flag is input from a printer engine employing the electrophotographic method, or times (T1, T2, and T3) at which an interruption signal is input to a control system. In most cases, the flag/interruption signal may be output at times at which a predetermined number of sheets are printed on. For example, in a case of a printing device in which density correction needs to be performed for output images when the number of sheets that have been printed on reaches 500, the flag/interruption signal is generated by a printer engine every time printing of 500 sheets has finished.
Next, a flow showing performance of the calibration process is described with reference to FIG. 10A. When a predetermined number of sheets have been printed on, a printer engine outputs a flag/interruption signal to a control system (hereinafter, referred to as a “controller”) of a printing device (step S501). When the controller receives the flag/interruption flag (YES in step S501), the controller issues an instruction for outputting, to the printer engine, data items of patch images that the printer engine has (step S502). The data items of patch images are image data items having a predetermined size/predetermined density level that are defined for each printer engine. Generally, the data items of patch images are defined as image data items having a plurality of patch-image density levels. FIGS. 11A to 11D are diagrams showing examples of configurations of patch images for a black-and-white printer engine. In FIGS. 11A to 11D, the patch images having four patch-image density levels are defined using H pixels in the main scanning direction, and V pixels in a sub-scanning direction. Patch images defined for a color printer engine are provided for each of components that form a color image. As one typical example, a plurality of patch images having predetermined patch-image density levels are provided for each of cyan (hereinafter, referred to as “C”), magenta (hereinafter, referred to as “M”), yellow (hereinafter, referred to as “Y”), and black (hereinafter, referred to as “B”) components.
When the calibration process is performed, patch images are formed at predetermined positions of a photoconductor drum. FIG. 12 shows an example in which patch images are formed on a photoconductor drum. Patch images 1202 that are formed on a photoconductor drum 1201 are not transferred onto transfer paper, which is different from a case in which a normal printing process is performed. Density values of the patch images are read by a sensor 1203 shown in FIG. 12 (step S503). If the density values read by the sensor 1203 are density values corresponding to the patch-image density levels of the patch images, there is no problem. However, in the electrophotographic method, density shifts occur in an output density characteristic when printing of a predetermined number of sheets has finished. For example, a read density value of a patch image having a patch-image density level of 64 may be much higher/lower than a density value corresponding to a density level of 64. The density shifts change in accordance with an influence of an environmental element, such as temperature or humidity of the inside/outside of the printing device, and in accordance with the amount of toner that has been consumed in order to print a predetermined number of sheets. In other words, a density shift of a patch image after an image data item that causes a sheet of white paper to be solidly colored has been printed on a predetermined number of sheets differs from a density shift after an image data item that has a large ratio of a white portion, such as a data item of a character image, has been printed on the predetermined number of sheets.
The calibration process in the electrophotographic method is performed to correct the density shifts. A density correction table that is used to the correct the density shifts is generated using the density values read by the sensor 1203, and the calibration process is performed using the density correction table (step S504).
FIGS. 13A to 13C illustrate a concept of generation of the density correction table. In FIGS. 13A to 13C, the horizontal axis indicates density levels including the patch-image density levels, and the vertical axis indicates output density levels obtained by reading the patch images formed on the photoconductor drum with the sensor 1203. Gridlines of the density levels on the horizontal axis correspond to the patch-image density levels. In this description, four types of patch-image density levels four types, i.e., patch-image density levels of 64, 128, 192, and 255, are provided. The patch images that are formed on photoconductor drum are read by the sensor 1203 to obtain read density values. The read density values are converted into density levels, and the density levels are shown as four points Pa, Pb, Pc, and Pd.
Because, in an ideal output density characteristic, the patch-image density levels are equal to density levels converted from the read density values obtained by the sensor 1203, the output density characteristic is a linear characteristic indicated by a curve W1. However, in most cases, an output density characteristic that exists after a predetermined number of sheets have been printed on is not plotted on the curve W1, as the points Pa to Pd show in FIG. 13A, and is plotted as output density levels that are shifted from the patch-image density levels of the respective patch images. In an example shown in FIG. 13A, the output density levels of the respective patch images are higher than the patch-image density levels plotted on the curve W1. In other words, even when the same image data item is used, a density level of an image obtained using the image data item after a predetermined number of sheets have been printed on, which corresponds to a time in which the calibration process is performed, is higher than a density level of an image obtained using the image data item when printing was started. Output density levels corresponding to density levels other than the patch-image density levels, for example, density levels 65 to 127, or 129 to 191, are determined by an interpolation process using output density levels corresponding to the patch-image density levels that are close to the density levels in most cases. In other words, output density levels corresponding to the density levels 65 to 127 are determined by an interpolation process using values of the points Pa and Pb. As a method of the interpolation process, a one-dimensional linear interpolation process may be used, or an interpolation process using a quadratic term may be used. By the interpolation process, an output density characteristic indicated by a curve W2 is obtained as the correlation between density levels that exist after a predetermined number of sheets have been printed on and output density levels.
By using the curve W2 as a result, the controller of the printing device generates a density correction table that is an inverse-transformation table indicated by a curve W3. The density correction table causes the output density characteristic indicated by the curve W2 that exists after the predetermined number of sheets have been printed on to be transformed into the output density characteristic indicated by the curve W1. An image data item that is input to the printing device is corrected by applying the density correction table indicated by the curve W3 to the image data item, whereby an output image having the linear density characteristic indicated by the curve W1 shown in FIG. 13B can be obtained.
When the density correction table that is the inverse-transformation table indicated by the curve W3 is set, an output density characteristic is obtained as the linear density characteristic indicated by the curve W1 by using the density correction table. However, then, when a predetermined number of sheets have been further printed on, density shifts occur again (FIG. 13C).
FIG. 14 is a graph showing time and density shift of an output image. The horizontal axis indicates time, and the vertical axis indicates output density level. Referring to FIG. 14, a state Ps is a state in which the calibration process was performed immediately before the state Ps, and a state Pe is a state in which printing of a predetermined number of sheets has finished (a time T0). A state Pr is a state in which the calibration process is performed while a job is being performed, and in which the calibration process has been completed (a time T1).
As shown in FIG. 14, a shift of the output density level occurs between the state Ps, which is a state in which the calibration process was performed immediately before the state Ps, and the state Pe, which is a state in which printing of a predetermined number of sheets has finished, corresponding to the time T0. In other words, a noticeable density shift exists between a quality level of an output image that exists immediately after the calibration process was performed and a quality level of an output image that exists immediately before the calibration process is next performed. The density shift (the slope corresponding to the density shift) depends on characteristics of an image that has been printed between the state Ps and the state Pe. In other words, a density shift that exists after an image having a high image ratio, such as a solidly colored image, has been printed on a larger number of sheets differs from a density shift that exists after an image having a low image ratio, which is an image having a large white region, such as a character image, has been printed on a larger number of sheets.
As described above, a process of correcting density shifts after a predetermined number of sheets have been printed on is the calibration process. However, because of a structure of the calibration process, a printing job that is being performed may need to be interrupted while the calibration process is being performed. Interruption of the printing job that is being performed directly leads to a decrease in productivity of the printing device. When a predetermined number of sheets to be printed on before the calibration process is next performed is increased in order to increase the productivity, the productivity is increased. However, the difference between a quality level of an output image that exists immediately after the calibration process was performed and a quality level of an output image that exists before the calibration process is next performed, i.e., the quality level difference (the density level difference), is increased.
Both an increase in productivity and outputting of a high-quality images may be simultaneously required for printing devices that generate, for example, high-quality printing materials in the “digital commercial printing” market. Additionally, in most cases, the printing devices suitable for the “digital commercial printing” market have performances such as a performance in which the number of sheets per minute to be printed on exceeds 100, and printer engines capable of outputting high-quality images are employed in the printing devices.
In order to achieve outputting of high-quality images, density shifts may need to be decreased as much as possible. In other words, ideally, the calibration process may need to be performed fairly often. However, because a printing job being performed needs to be interrupted while the calibration process is being performed, it may be difficult to meet the performance that may be necessary for high-speed devices for POD. Furthermore, the number of sheets per minute that the high-speed devices for POD can print is quite a few. In a case in which the calibration process is performed after a predetermined number of sheets have been printed on, even when a predetermined number of sheets to be printed on before the calibration process is next performed is 2000, the calibration process has typically been performed one time before twenty minutes elapses.
Regarding the decrease in productivity due to performance of the calibration process, Japanese Patent Laid-Open No. 2004-142163 discloses that a density correction process is switched on the basis of information concerning environmental temperature/humidity of the inside/outside of a printing device. FIG. 15 is a diagram showing times at which a calibration process is performed, which is disclosed in Japanese Patent Laid-Open No. 2004-142163. It is disclosed that, when environmental temperature/humidity changes (a time Tk), the calibration process is performed using outputting of patch images or generation of correction data on the basis of a simulation performed by a host device connected to the printing device is selected. The number of performances of the calibration process using outputting of patch images with the printing device is decreased by switching the density correction process on the basis of a change in environmental temperature/humidity of the inside/outside of the printing device, so that the productivity is increased.
Additionally, Laid-Open No. 11-177822 discloses that a time at which a calibration process is performed is changed in accordance with an image-output-operation mode set by a user who uses a printing device. FIG. 16 is a diagram showing times at which the calibration process is performed, which is disclosed in Laid-Open No. 11-177822. It is disclosed that, when an image-output-operation mode in which the calibration process using outputting of patch images does not need to be performed so often is selected by the user after a calibration time T0, the next calibration time T1 is delayed. In this manner, the number of performances of the calibration process using outputting of patch images with the printing device is decreased, so that the productivity is increased.
Laid-Open No. 11-164148 discloses correlation among an operation mode of the calibration process set by a user who uses a printing device, a calibration process, and checking of a quality level of a predicted output image. More specifically, it is disclosed that, it is recognized that the calibration process is performed while a job is being performed, and disclosed that a quality level of the final image to be provided in the job can be checked before the job is performed. FIG. 17 is a diagram showing times at which the calibration process is performed and times at which a quality level of a predicted output image is checked, which is disclosed in Laid-Open No. 11-164148.
In Laid-Open No. 11-164148, a case is described in which three types of operation modes of the calibration process of the printing device, i.e., a high-speed operation mode in which 300 sheets are to be printed on before the calibration process is next performed, a normal speed operation mode in which 150 sheets are to be printed on before the calibration process is next performed, and a high-resolution operation mode in which 50 sheets are to be printed on before the calibration process is next performed, are provided. In a case in which the normal operation mode is set as an operation mode of the calibration process when the user starts using the printing device, and in which, when a job is input, the number of sheets to be printed on that is specified in the job is 200, the calibration process is performed after 150 sheets have been printed on (T0). Accordingly, the last image data item provided in the job is an image data item provided at a time (Tjbe) at which another 50 sheets were printed on after density correction was performed by the calibration process to obtain a linear density characteristic. Laid-Open No. 11-164148 discloses that a density-shift operation coefficient provided at a time at which one sheet was printed on is maintained in advance, that a quality level of an output image is predicted by a proportional calculation, and that the predicted output image is displayed. In other words, in the example described above, the value 50 that is the number of sheets to be printed on after the calibration process is performed is multiplied by the density-shift operation coefficient to generate a predicted output image. The user checks a quality level of the predicted output image before the job is started, and selects/switches the operation mode (the high-speed/normal-speed/high-resolution operation mode) of the calibration process of the printing device.
Regarding the disclosure in Laid-Open No. 2004-142163, it is described that the number of performances of the calibration process using outputting of patch images is reduced in order to improve the productivity. Only an environmental temperature/humidity parameter is focused on for the density correction process. However, as already described, not only the environmental temperature/humidity parameter but also a parameter of an image ratio (an image density level) may be important as a factor of a density shift of an output image that exists after a predetermined number of sheets have been printed on. Furthermore, what type of density correction table is specifically generated using the environmental temperature/humidity parameter in order to perform the density correction process is not described.
In the disclosure of Laid-Open No. 11-177822, a time at which the calibration process is performed can be changed in accordance with the image-output-operation mode. Because a time at which the calibration process is performed is delayed, the productivity is increased. However, a time at which density correction is performed is delayed. Thus, regarding a quality level of an output image in reality, it may be difficult to achieve outputting of a high-quality image, and it may be necessary to allow degradation in quality.
In the disclosure of Laid-Open No. 11-164148, an auxiliary function that a user may use to set the operation mode of the calibration process before a job is started is provided. A density correction process for improving a quality level of an output image is not provided. Additionally, a quality level of the final output image to be provided in a job is predicted and output. However, because a parameter used for prediction has a fixed value, an influence of an image data item that has been printed on before the calibration process is next performed is not considered.
In other words, in the disclosures described above, no solution is particularly provided to achieve both an increase in productivity and an improvement in quality level of an output image that may be necessary for high-speed devices for POD.